The invention relates to a hydraulic selector valve assembly for marine steering systems, particularly steering systems for relatively large marine vessels which utilize at least two power cylinders actuating a single rudder or coupled twin rudders.
Marine safety certifying authorities such as Lloyds of London or the American Bureau of Shipping require that a steering system normally utilizing two power cylinders for controlling single or twin rudders must be able to operate in a single cylinder steering mode, should there be a failure of the remaining cylinder or circuits associated with that cylinder. Thus, for safety reasons, the steering system must be operable with one cylinder active, and the other cylinder inactive. Usually, the inactive or passive cylinder can be made to "float" as it is driven by the active or operable cylinder which enables functioning of the steering gear with one cylinder only. However, because most large marine steering systems use unbalanced cylinders, that is the piston rod associated with the cylinder does not pass through both ends of the cylinder, a different volume of fluid is displaced when the cylinder extends or retracts, i.e there is a fluid volume differential on opposite sides of the piston. The volume differential presents problems in an emergency when it is necessary to isolate the inactive cylinder in a closed circuit because when the active cylinder drives the inactive cylinder the volume differential of the inactive cylinder causes a hydraulic lock.
To enable operation of the steering system using one cylinder, six or eight manually actuated emergency valves are provided in appropriate hydraulic lines to enable an engineer to direct fluid in such a way that the active cylinder can steer the vessel, and the inactive cylinder can follow movement of the rudder without undue restriction. To enable the steering to function using the remaining cylinder, the engineer must manipulate the correct emergency valves in the correct sequence to direct fluid appropriately. However, because failure usually happens in extreme weather conditions or exceptional situations, which occur only rarely, the engineer is usually unfamiliar with the correct manipulation of the emergency valves, and thus mistakes often occur so that the emergency single cylinder steering mode is not always easily and quickly available.